This invention relates to electrochemical cells. More particularly, it is concerned with positive electrodes for rechargeable non-aqueous electrochemical cells.
Rechargeable non-aqueous electrochemical cells employing all inorganic sulfur dioxide electrolyte with lithium negative electrodes and metal halide positive electrodes have been developed. For so-called spiral wound cells, flexible positive electrodes are required. One method employed in fabricating flexible positive electrodes involves mixing the active material of the electrode with carbon and a suitable mechanical binder such as polytetrafluoroethlyene (TFE) in the form of an aqueous dispersion. The slurry formed by mixing the ingredients is applied to a metal screen-like substrate. This structure is pressed to remove the water, dried, and then heated to cure, or sinter, the binder.
One very useful active electrode material for use in the positive electrodes of rechargeable sulfur dioxide cells is cupric chloride (CuCl.sub.2). Cupric chloride, however, is water soluble. If in order to overcome the problem of its solubility, a saturated aqueous solution of cupric chloride is used, the final dried electrode contains a non-uniform distribution of exceptionally large cupric chloride crystals rather than a uniform distribution of small crystals. Positive electrodes with large cupric chloride crystals do not provide satisfactory cell operation.
Positive electrodes using water-soluble metal halides, such as cupric chloride, typically are prepared by pressing the anhydrous powder of the active material together with carbon and a binder, such as TFE, onto a substrate. Positive electrodes produced by this pressed powder technique have been found to be inflexible and tend to crumble when attempts are made to bend them to form spiral wound electrode assemblies. Furthermore, the positive electrode expands and contracts during the operation of a rechargeable electrochemical cell causing additional stress to develop on the mechanical structure of the positive electrode.